编译器对结构进行重新排序 [英] Struct Reordering by compiler

问题描述

假设我有一个这样的结构:

struct MyStruct
{
  uint8_t var0;
  uint32_t var1;
  uint8_t var2;
  uint8_t var3;
  uint8_t var4;
};

这可能会浪费一堆(不是一吨)的空间.这是因为uint32_t变量必须进行对齐.

实际上(在对齐结构以便可以实际使用uint32_t变量之后)，它看起来可能像这样:

struct MyStruct
{
  uint8_t var0;
  uint8_t unused[3];  //3 bytes of wasted space
  uint32_t var1;
  uint8_t var2;
  uint8_t var3;
  uint8_t var4;
};

更有效的结构是:

struct MyStruct
{
  uint8_t var0;
  uint8_t var2;
  uint8_t var3;
  uint8_t var4;
  uint32_t var1;
};

现在，问题是:

为什么(标准)禁止编译器对结构进行重新排序?

如果结构重新排序，我看不出有什么办法可以让自己在脚上开枪.

解决方案

为什么(按标准)禁止编译器对结构进行重新排序?

基本原因是:与C兼容.

请记住，C最初是一种高级汇编语言.在C语言中，通过将字节重新解释为特定的struct来查看内存(网络数据包等)是很常见的.

这导致了依赖此属性的多个功能:

• C保证struct的地址和它的第一个数据成员的地址是相同的，因此C ++也是如此(在没有virtual继承/方法的情况下).

• C保证如果您有两个struct A和B，并且都以一个数据成员char开头，然后是一个数据成员int(以及之后的任何其他字符)，那么当您将它们放在union中，您可以编写B成员并通过其A成员读取char和int，因此C ++也会这样做: 解决方案

Why is the compiler forbidden (by the standard) from reordering the struct?

The basic reason is: for compatibility with C.

Remember that C is, originally, a high-level assembly language. It is quite common in C to view memory (network packets, ...) by reinterpreting the bytes as a specific struct.

This has led to multiple features relying on this property:

• C guaranteed that the address of a struct and the address of its first data member are one and the same, so C++ does too (in the absence of virtual inheritance/methods).

• C guaranteed that if you have two struct A and B and both start with a data member char followed by a data member int (and whatever after), then when you put them in a union you can write the B member and read the char and int through its A member, so C++ does too: Standard Layout.

The latter is extremely broad, and completely prevents any re-ordering of data members for most struct (or class).

---

Note that the Standard does allow some re-ordering: since C did not have the concept of access control, C++ specifies that the relative order of two data members with a different access control specifier is unspecified.

As far as I know, no compiler attempts to take advantage of it; but they could in theory.

Outside of C++, languages such as Rust allow compilers to re-order fields and the main Rust compiler (rustc) does so by default. Only historical decisions and a strong desire for backward compatibility prevent C++ from doing so.

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